Yes, you can light a candle if the glass is cracked, but it's not recommended. The crack weakens the glass, making it more likely to shatter completely from the heat of the flame. This could cause hot wax to spill, posing a fire hazard or causing burns.
Signs that candle glass might be about to crack include visible new hairline fractures appearing, the glass feeling excessively hot to the touch, or hearing faint popping or creaking sounds. If you notice any of these indicators, extinguish the candle immediately and allow it to cool completely before safely discarding it to prevent further damage or injury.